2,060 research outputs found
MolMod – an open access database of force fields for molecular simulations of fluids
The MolMod database is presented, which is openly accessible at http://molmod.boltzmann-zuse.de and contains intermolecular force fields for over 150 pure fluids at present. It was developed and is maintained by the Boltzmann-Zuse Society for Computational Molecular Engineering (BZS). The set of molecular models in the MolMod database provides a coherent framework for molecular simulations of fluids. The molecular models in the MolMod database consist of Lennard-Jones interaction sites, point charges, and point dipoles and quadrupoles, which can be equivalently represented by multiple point charges. The force fields can be exported as input files for the simulation programmes ms2 and ls1 mardyn, GROMACS, and LAMMPS. To characterise the semantics associated with the numerical database content, a force field nomenclature is introduced that can also be used in other contexts in materials modelling at the atomistic and mesoscopic levels. The models of the pure substances that are included in the database were generally optimised such as to yield good representations of experimental data of the vapour–liquid equilibrium with a focus on the vapour pressure and the saturated liquid density. In many cases, the models also yield good predictions of caloric, transport, and interfacial properties of the pure fluids. For all models, references to the original works in which they were developed are provided. The models can be used straightforwardly for predictions of properties of fluid mixtures using established combination rules. Input errors are a major source of errors in simulations. The MolMod database contributes to reducing such errors.BMBF, 01IH16008E, Verbundprojekt: TaLPas - Task-basierte Lastverteilung und Auto-Tuning in der PartikelsimulationEC/H2020/694807/EU/Enrichment of Components at Interfaces and Mass Transfer in Fluid Separation Technologies/ENRICOEC/H2020/760907/EU/Virtual Materials Market Place (VIMMP)/VIMM
Observation of Three-dimensional Long-range Order in Smaller Ion Coulomb Crystals in an rf Trap
Three-dimensional long-range ordered structures in smaller and
near-spherically symmetric Coulomb crystals of ^{40}Ca^+ ions confined in a
linear rf Paul trap have been observed when the number of ions exceeds ~1000
ions. This result is unexpected from ground state molecular dynamics (MD)
simulations, but found to be in agreement with MD simulations of metastable ion
configurations. Previously, three-dimensional long-range ordered structures
have only been reported in Penning traps in systems of ~50,000 ions or more.Comment: 5 pages; 4 figures; to appear in Phys. Rev. Lett.; changed content
From finite nuclei to the nuclear liquid drop: leptodermous expansion based on the self-consistent mean-field theory
The parameters of the nuclear liquid drop model, such as the volume, surface,
symmetry, and curvature constants, as well as bulk radii, are extracted from
the non-relativistic and relativistic energy density functionals used in
microscopic calculations for finite nuclei. The microscopic liquid drop energy,
obtained self-consistently for a large sample of finite, spherical nuclei, has
been expanded in terms of powers of A^{-1/3} (or inverse nuclear radius) and
the isospin excess (or neutron-to-proton asymmetry). In order to perform a
reliable extrapolation in the inverse radius, the calculations have been
carried out for nuclei with huge numbers of nucleons, of the order of 10^6. The
Coulomb interaction has been ignored to be able to approach nuclei of arbitrary
sizes and to avoid radial instabilities characteristic of systems with very
large atomic numbers. The main contribution to the fluctuating part of the
binding energy has been removed using the Green's function method to calculate
the shell correction. The limitations of applying the leptodermous expansion to
actual nuclei are discussed. While the leading terms in the macroscopic energy
expansion can be extracted very precisely, the higher-order, isospin-dependent
terms are prone to large uncertainties due to finite-size effects.Comment: 13 pages revtex4, 7 eps figures, submitted to Phys. Rev.
Amplitude analysis of four-body decays using a massively-parallel fitting framework
The GooFit Framework is designed to perform maximum-likelihood fits for
arbitrary functions on various parallel back ends, for example a GPU. We
present an extension to GooFit which adds the functionality to perform
time-dependent amplitude analyses of pseudoscalar mesons decaying into four
pseudoscalar final states. Benchmarks of this functionality show a significant
performance increase when utilizing a GPU compared to a CPU. Furthermore, this
extension is employed to study the sensitivity on the mixing
parameters and in a time-dependent amplitude analysis of the decay . Studying a sample of 50 000 events and setting
the central values to the world average of and , the statistical sensitivities of and are determined
to be and .Comment: Proceedings of the 22nd International Conference on Computing in High
Energy and Nuclear Physics, CHEP 201
A Structure Function Model Recovers the Many Formulations for Air-Water Gas Transfer Velocity
Two ideas regarding the structure of turbulence near a clear air-water interface are used to derive a waterside gas transfer velocity k(L) for sparingly and slightly soluble gases. The first is that k(L) is proportional to the turnover velocity described by the vertical velocity structure function D-ww(r), where r is separation distance between two points. The second is that the scalar exchange between the air-water interface and the waterside turbulence can be suitably described by a length scale proportional to the Batchelor scale l(B) = Sc-1/2, where Sc is the molecular Schmidt number and eta is the Kolmogorov microscale defining the smallest scale of turbulent eddies impacted by fluid viscosity. Using an approximate solution to the von Karman-Howarth equation predicting D-ww(r) in the inertial and viscous regimes, prior formulations for k(L) are recovered including (i) kL = root 2/15Sc(-1/2)v(K), v(K) is the Kolmogorov velocity defined by the Reynolds number v(K)eta/nu = 1 and nu is the kinematic viscosity of water; (ii) surface divergence formulations; (iii) k(L) alpha Sc(-1/2)u(*), where u(*) is the waterside friction velocity; (iv) k(L) alpha Sc-1/2 root g nu/u(*) for Keulegan numbers exceeding a threshold needed for long-wave generation, where the proportionality constant varies with wave age, g is the gravitational acceleration; and (v) k(L) = root 2/15Sc(-1/2) (nu g beta(o)q(o))(1/4) in free convection, where q(o) is the surface heat flux and beta(o) is the thermal expansion of water. The work demonstrates that the aforementioned k(L) formulations can be recovered from a single structure function model derived for locally homogeneous and isotropic turbulence.Peer reviewe
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